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I'm expanding my deck and will be replacing the PT decking with 5/4" x6" mahagony. We live in a historic section of Cape Cod and composite materials aren't allowed. The deck is south facing and gets sun all day long. Question is how should I treat the deck for maximum life with as but with low effort? Also, suggestions for fastening would be appreciated.

How you treat your deck depends on your desired end result. If you want low maintenance and a grey/silver deck, leave it unfinished. You will still want to clean (wash with TSP and H2O) to prevent build up of dirt and surface mold. If you are looking for it to maintane the look it has the day you installed it, then a good oil based stain is in your future. Cabots or Penofin both make products specifically formulated for exotic hardwoods. Keep in mind they are only slowing down what the UV rays are doing to the wood (causing it to fade). This will be a yearly process for you. You should either use a hidden fastener system like Tiger Claw TCG or TC120 (depending on the thickness of your decking) or use a stainless steel trim head screw. Use a 316 grade if you are near salt water. You will need to predrill for the screws to avoid splitting the ends or stripping the screws in the field.

Hi,
We use mahogany for porch flooring, but not in a fully-exposed deck installation. In your case you have a rather extreme exposure with snow, salt water/mist, and hi UV. I would only consider IPE for a situation like yours. And just let it weather. IPE turns a very gentle gray in about 2 years, just like cedar shingles. For reference, see the boardwalk at Atlantic City.
All of the factors against lesser woods are also brought to bear against any type of clear finish or stain that you would strive to maintain; a losing proposition I'm afraid unless applied yearly.
S_M